Molecular-strain induced phosphinidene reactivity of a phosphanorcaradiene

Phosphanorcaradienes are an appealing class of phosphorus compounds that can serve as synthons of transient phosphinidenes. However, the synthesis of such species is a formidable task owing to their intrinsic high reactivity. Herein we report straightforward synthesis, characterization and reactivity studies of a phosphanorcaradiene, in which one of the benzene rings in the flanking fluorenyl substituents is intramolecularly dearomatized through attachment to the phosphorus atom. It is facilely obtained by the reduction of phosphorus(III) dichloride precursor with potassium graphite. Despite being thermally robust, it acts as a synthetic equivalent of a transient phosphinidene. It reacts with trimethylphosphine and isonitrile to yield phosphanylidene-phosphorane and 1-phospha-3-azaallene, respectively. When it is treated with one and two molar equivalents of azide, iminophosphane and bis(imino)phosphane are isolated, respectively. Moreover, it is capable of activating ethylene and alkyne to afford [1 + 2] cycloaddition products, as well as oxidative cleavage of Si–H and N–H bonds to yield secondary phosphines. All the reactions proceed smoothly at room temperature without the presence of transition metals. The driving force for these reactions is most likely the high ring-constraint of the three-membered PC2 ring and recovery of the aromaticity of the benzene ring.

Phosphanorcaradienes are an appealing class of phosphorus compounds that can serve as synthons of transient phosphinidenes.However, the synthesis of such species is a formidable task owing to their intrinsic high reactivity.Herein we report straightforward synthesis, characterization and reactivity studies of a phosphanorcaradiene, in which one of the benzene rings in the flanking fluorenyl substituents is intramolecularly dearomatized through attachment to the phosphorus atom.It is facilely obtained by the reduction of phosphorus(III) dichloride precursor with potassium graphite.Despite being thermally robust, it acts as a synthetic equivalent of a transient phosphinidene.It reacts with trimethylphosphine and isonitrile to yield phosphanylidene-phosphorane and 1-phospha-3-azaallene, respectively.When it is treated with one and two molar equivalents of azide, iminophosphane and bis(imino)phosphane are isolated, respectively.Moreover, it is capable of activating ethylene and alkyne to afford [1 + 2] cycloaddition products, as well as oxidative cleavage of Si-H and N-H bonds to yield secondary phosphines.All the reactions proceed smoothly at room temperature without the presence of transition metals.The driving force for these reactions is most likely the high ring-constraint of the threemembered PC 2 ring and recovery of the aromaticity of the benzene ring.
In contrast to phosphepines, phosphanorcaradienes, which are phosphorus analogs of NCDs, have higher molecular-constraints and thereby exhibit inherent high reactivity, making their isolation in the condensed state a formidable task.On this basis, such compounds may be ideal synthons for accessing transient phosphinidenes via phosphorus-type retro-Büchner reactions.However, only one free phosphanorcaradiene has been reported by Stephan and coworkers.It was synthesized through demetalation of a phosphepinegold complex, while its reactivity as a phosphinidene precursor for small molecule activation has not been fully disclosed 42 .Very recently, a ruthenophosphanorcaradiene acting as a synthon for an ambiphilic metallophosphinidene was reported by Scheer, Tilley and coworkers 43 .
Our group has continuous research interests in synthesizing lowcoordinate main-group species, and has successfully isolated and structurally characterized several heavier analogs of free carbynes [44][45][46][47] , triplet stibinidene and bismuthinidenes (Fig. 2a) [48][49][50] supported by sterically encumbered hydrindacene ligands 51 .Encouraged by these results, we continued our research to pursue isolable phosphinidenes.In this contribution, we report straightforward synthesis, characterization and reactivity studies of a phosphanorcaradiene 1 (Fig. 2b).Reactivity studies reveal that it can serve as an elegant synthon of phosphinidene due to the release of molecular-strain 52,53 .

Synthesis and characterization of 1
With the aim of synthesizing a stable phosphinidene, we carried out the reduction of the phosphorus(III) dichloride M s Fluid tBu -PCl 2 54 with two molar equivalents of potassium graphite in THF; however, phosphanorcaradiene 1 was obtained in 75% yield as a yellow solid instead of the expected product (Fig. 3a).One of the benzene rings in the flanking fluorenyl moieties is intramolecularly dearomatized through attachment to the phosphorus atom.Contrastingly, similar reduction reactions with less sterically hindered ligands afforded diphosphenes or other higher oligomers [55][56][57] .The formation of the PC 2 threemembered ring leads to a decrease in the symmetry and complex 1 H and 13 C{ 1 H} NMR spectra.The proton signal in the PC 2 ring was shown at δ 2.54 ppm.A sharp singlet signal at δ-155.1 ppm was observed in the 31 P{ 1 H} NMR spectrum in C 6 D 6 solution, similar to that of phosphiranes 22,58 .Interestingly, 1 can be heated to 100 o C in C 6 D 6 solution for one hour without noticeable decomposition under an inert atmosphere, but it is highly air-and moisture-sensitive and yields intractable mixtures when exposed to air or dry oxygen.The synthesis of 1 is more straightforward in comparison to that of the phosphanorcaradiene reported by Stephan and coworkers. [18]ingle crystals of 1 suitable for X-ray diffraction analysis were obtained by layering n-hexane in a toluene solution at 4 o C 59 .It crystallizes in the monoclinic space group P1.There are two independent enantiomers present in one crystal unit cell due to the chirality of the P atom.One of the molecules is shown in Fig. 3b, which unambiguously reveals the connection of the P atom to two C atoms of the flanking fluorenyl group.The distances of P1-C27 (1.973(5) Å) and P1-C28 (1.985(5) Å) are substantially longer than those of P1-C1 (1.847(4) Å) and the P-C distances of the PC 2 ring in phosphiranes 22,58 .Consistently, the C27-P1-C28 angle (43.78(19) o ) is more acute.Moreover, the P atom deviates from the position expected for atoms attached to a phenyl group as evidenced by the large difference between the bond angles of C2-C1-P1 (111.7(3)o ) and C6-C1-P1 (131.7(4)o ).These data suggest that the PC 2 ring in 1 bears a greater ring-strain than phosphiranes.

Theoretical calculations of 1
The electronic structure of 1 was further elucidated by density functional theory (DFT) calculations at the BP86 + D3BJ/def2-SVP level.The HOMO represents the bonding orbitals of the C 2 P ring and the C-C πorbitals of the linked C 6 ring (Fig. 3c-e).The lone pair of P atom is shown in the HOMO-4, and the LUMO contains the π * -antibonding orbitals of the fluorenyl moiety linked to the P atom.Additionally, natural bond orbital (NBO) and intrinsic bond orbital (IBO) 60 analyses were conducted to gain insight into the bonding character of 1.The Wiberg bond indices (WBIs) of the P1-C27 (0.74) and P1-C28 (0.72) bonds are substantially smaller than that of the P1-C1 bond (0.96), suggesting that they are relatively weaker than the P1-C1 bond.Moreover, they are smaller than those of the phosphanorcaradiene reported by Stephan and coworkers (0.79 and 0.76), implying a more fragile PC 2 ring in 1. IBO analysis shows that P1-C27(C28) σ-bonds are formed by the overlap of the 3p orbitals at P1 and the 2p orbitals at C27(C28) (Supplementary Fig. 1).The 3p orbitals at the P atom and the 2p orbitals at the C atoms provided more than 90% of the bonding electrons in the P1-C27(C28) bonds.These orbitals are significantly different from the P1-C1 σ-bond, which is formed by the 3p orbital with the partial s character of P1 and the sp 2 hybrid orbital of C1.
To further explore the electronic structure of the C 2 P ring of 1, the quantum theory of atoms in molecules (QTAIM) and electron localization function (ELF) analyses were carried out 61,62 .The QTAIM calculations indicate that there are three bond critical points (BCPs) among the P1, C27 and C28 atoms (Supplementary Fig. 2).In addition, the bond paths between P1 and C27(C28) slightly deviate away from the P1-C27(C28) axes.Furthermore, the ELF plots exhibit a twisted region with highly localized electron density between each C and P in the C 2 P ring (Supplementary Fig. 3).
The formation of 1 can be simplified to [2 + 1] cycloaddition of a transient phosphinidene with a C = C double of the flanking fluorenyl group.We previously reported the isolation of triplet stibinidene 48 and bismuthinidenes 49 , which are heavy congeners of the proposed phosphinidene.A comparison of the calculated energies between triplet phosphinidene and 1 showed that 1 is 5.7 kcal/mol lower in energy than that of triplet phosphinidene (Supplementary Fig. 4).The conversion of triplet phosphinidene to 1 is facile with a predicted barrier no more than 5 kcal/mol.
To provide further understanding of 1, electronic and steric properties were theoretically studied.The proton affinity was analyzed to evaluate the electronic property, and meanwhile related calculations on PPh 3 and Mes-P(CH 2 CH 2 ) were also performed for comparison.As shown in Supplementary Fig. 5, the proton affinity of 1 is -156.8kcal/mol, which is lower than those of PPh 3 (-152.8kcal/mol) and Mes-P(CH 2 CH 2 ) (-141.8 kcal/mol).Compared with PPh 3 and the analogous three-membered ring phosphirane Mes-P(CH 2 CH 2 ), 1 has a better capability of electron donating.In addition, the steric property of 1 were studied with percent buried volume (%V bur ) analysis.The % V bur of 1 is 76.6%, higher than those of PPh 3 (65.0%)and Mes-P(CH 2 CH 2 ) (54.3%).The results of percent buried volumes suggest that the steric hindrance around the phosphorus atom of 1 is greater than those of PPh 3 and Mes-P(CH 2 CH 2 ).Besides, steric maps shown in Supplementary Fig. 5 also display a larger steric hindrance of 1 than PPh 3 and Mes-P(CH 2 CH 2 ).Owing to the steric effect, this active phophanorcaradiene species 1 can be stabilized.

Reactions of 1 with alkene and alkyne
The ability of 1 to activate alkene and alkyne was further investigated (Fig. 6).Exposure of 1 to 1 atm. of ethylene atmosphere at room  temperature led to the formation of phosphirane 7 in moderate yield.The 31 P{ 1 H} NMR resonance was shown at δ -229.2 ppm, similar to that of 1-mesitylphosphirane (δ -238.9 ppm) 72 .Complete consumption of 1 with an excess amount of 4-tertbutylphenylacetylene at room temperature was observed after 12 h, and phosphirene 8 was isolated in 65% yield.A single resonance signal was observed at δ -171.3 ppm in the 31 P{ 1 H} NMR spectrum of 8.The formation of 7 and 8 under mild conditions is most likely attributed to the decrease in the ring-strain and recovery of aromaticity at the six-membered ring in 1, which is striking since phosphinidene transfer to alkenes with II and IV developed by the Cummins group has to be promoted by heating or catalyzed by TM catalysts 32,34,35 .It is noteworthy that 7 and 8 do not react with 4-tertbutylphenylazide, most probably attributed to high steric hindrance around the phosphorus atoms.
The molecular structures of 7 and 8 determined by SC-XRD analysis are shown in Fig. 7. New three-membered PC 2 rings are shown in the structures.The C57-C58 distances are 1.513(4) and 1.308(5) Å, respectively, in accordance with their single and double bond nature.Moreover, the P-C bond lengths (1.847(3) and 1.848(3) Å for 7; 1.779(4) and 1.808(4) Å for 8) inside the PC 2 ring are shortened in comparison to those in 1.In contrast to that of 1, the P atoms in 7 and 8 are in the expected positions for atoms attached to a phenyl ring.

Activation of Si-H and N-H bonds with 1
Recently, geometrically constrained phosphines have shown to be capable of σ-bond activation [73][74][75][76] , prompting us to study the ability of 1 in σ-bond activation.Interestingly, we found that 1 could activate the Si-H and N-H bonds.The reactions of 1 with diethylsilane and phenylamine occurred smoothly at room temperature, and silylphosphine 9 and aminophosphine 10 were obtained in 70% and 74% yields, respectively (Fig. 6).Their structures were unambiguously determined by SC-XRD analysis (Fig. 7).The activation of Si-H and N-H bonds represents rare examples of intermolecular inert bond activation with metal-free transient phosphinidenes.Oxidation cleavage of Si-H bonds with electrophilic metallo-phosphinidene complexes was reported by Sterenberg et al. 77,78 .Stephan and coworkers showed hydrosilylation of low-valent P 5 Ph 5 mediated by the Lewis acid B(C 6 F 5 ) 3

79
. Hering-Junghans and coworkers recently revealed the first example of N-H activation at a metal-free P(I) center with phospha-Wittig reagents 66 .Additionally, the reactions of 1 with dioxygen and dihydrogen led to intractable mixtures.

Theoretical studies of the reaction mechanisms
The mechanisms for the activation of alkene, alkyne, silane, and amine with 1 were studied by DFT calculations.For alkene, alkyne, and silane, as shown in Fig. 8a-c, respectively, the breakage of the PC 2 threemembered ring structures and activations of the C = C bond, the C ≡ C bond, and the Si-H bond occur concertedly in respective transition states (7-TS, 8-TS, and 9-TS, respectively).Through respective transition states, the addition products 7, 8, and 9 are formed.While for the activation of amine, the phosphorus atom interacts with amine to form 10-RC firstly, in which the three-membered ring structure in 1 is broken (Fig. 8d).The N-H bond is activated through 10-TS to form product 10.The energy barriers for activations of alkene, alkyne, silane, and amine are 15.1, 11.4, 7.4, and 21.1 kcal/mol, respectively, which are in line with the reaction conditions of room temperature.As shown in Fig. 8, these transformations are exothermic processes through early transition states, and thus the activations of alkene, alkyne, silane, and amine are facile by the active phophanorcaradiene 1.
In addition, the structures of 7-TS, 8-TS, 9-TS, and 10-TS were analysed.As Fig. 9 shows, P-C 1 and P-C 2 distances in these transition states are all larger than those in 1 (1.980 and 2.002 Å for P-C  typical concerted asynchronous transition states.For 7-TS, the P-C 3 and the P-C 4 distances are 2.552 and 2.871 Å, respectively, and for 8-TS, the P-C 3 and the P-C 4 distances are 3.137 and 2.239 Å respectively.For 9-TS, the Si-H distance is 1.543 Å, and the N-H distance in 10-TS is 1.249 Å, suggesting the activations of the Si-H bond and the N-H bond.Furthermore, distortion analysis was performed on 7-TS, 8-TS, 9-TS, and 10-TS 80,81 .In the distortion analysis, these transition states were divided into the activated molecule moiety and the phophanorcaradiene moiety.The total deformation energies (ΔE dist ) of 7-TS, 8-TS, 9-TS, and 10-TS are shown in Fig. 9.The deformations of 7-TS, 8-TS, and 9-TS compared to respective RCs are not as large as that of 10-TS, probably due to the aromatization driving force of the decyclization of P-ligand three-membered ring structure in the transition state.The ΔE dist of 7-TS, 8-TS, and 9-TS are 19.0,17.8, and 18.6 kcal/mol respectively, which are all lower than that of 10-TS (47.9 kcal/mol).Therefore, the energy barrier of the activation of amine is relatively higher than those of activations of alkene, alkyne, and silane.In addition, the interaction energies (ΔE int ) of transition states were also analyzed.ΔE int of 10-TS is -44.5 kcal/mol, which is much higher than those of 7-TS, 8-TS, and 9-TS (-11.1, -19.5, and -22.5 kcal/mol, respectively).
Although the ΔE dist of 10-TS is high, the high ΔE int of 10-TS leads to a relatively low energy barrier for amine activation.

Discussion
In summary, we have described the synthesis and characterization of thermally robust phosphanorcaradiene 1 with high molecular-strain.It could react with nucleophiles and electrophiles.Strikingly, 1 is capable of activating C = C double and C ≡ C triple bonds, as well as Si-H and N-H bonds at room temperature.The driving force for these reactions is most likely the high ring-strain of the three-membered PC 2 ring and recovery of aromaticity of the C 6 ring.This work demonstrates the great potential of phosphanorcaradiene as a synthetic equivalent of phosphinidene in small molecule activation.The use of 1 to synthesize more phosphorus compounds including phosphinidene-TM catalysts is under investigation in our laboratory.

Methods
All experiments were carried out under a dry oxygen-free nitrogen atmosphere using standard Schlenk techniques or in a N 2 filled-glove box.Solvents were dried by standard methods and stored in activated 4 Å molecule sieve in the glovebox.All reagents were purchased from commercial sources (Energy Chemical and TCI) and used without further purification unless otherwise noted.M s Fluind t Bu-PCl 2

54
, and KC 8 82 were synthesized according to reported procedures.The 1 H, 13 C{ 1 H} and 31 P{ 1 H} NMR spectra were recorded on Bruker spectrometers (AV400 and AV600).Chemical shift values for protons are referenced to the residual proton resonance of CDCl 3 (δ: 7.26), C 6 D 6 (δ: 7.16), THF-d 8 (δ: 3.62); chemical shift values for carbons are referenced to the carbon resonance of CDCl 3 (δ: 77.16), C 6 D 6 (δ: 128.06),THF-d 8 (δ: 67.21); chemical shift values for phosphorus are relative to 85% H 3 PO 4 .NMR multiplicities are abbreviated as follows: s = singlet, d = doublet, t = triplet, q = quartet, sept = septet, m = multiplet, br = broad signal.Chemical shifts are quoted in δ (ppm) and coupling constants in Hz.The samples were dissolved in deuterated solvents, and were sealed off in J-Young NMR tubes for measurements.For the single crystal X-ray structure analysis, the crystals were each mounted on a glass capillary in perfluorinated oil and measured in a cold N 2 flow.The data for all compounds were collected on a Bruker D8 Venture or XtaLAB Synergy R, DW system, HyPix diffractometer at low temperatures.

2 Fig. 1 |
Fig. 1 | Büchner ring expansion and selected P(III) compounds that act as synthons of transient phosphinidenes.a Schematic depiction of Büchner ring expansion and the equilibria between cycloheptatriene (CHT) and norcaradiene (NCD); R is organic substituent.b Selected examples of P(III) precursors as synthons of transient phosphinidenes; R is organic substituent, and L is coordinating ligand.

Fig. 5 |
Fig. 5 | Molecular structures of 2-4 and 6.Thermal ellipsoid drawings of the molecular structures of 2 (a), 3 (b), 4 (c) and 6 (d) at 50% probability.All hydrogen atoms are omitted, and the fluorenyl moieties and the 4-tBuC 6 H 4 groups are shown in a wireframe style for clarity.

Fig. 7 |
Fig. 7 | Molecular structures of 7-10.Thermal ellipsoid drawings of the molecular structures of 7 (a), 8 (b), 9 (c) and 10 (d) at 50% probability.All hydrogen atoms except those at the P, Si and N atoms in 7 and 8 are omitted, and the fluorenyl moieties, the 4-tBuC 6 H 4 and phenyl groups are shown in a wireframe style for clarity.